(a) Field of the Invention
The present invention relates in general to satellite communications and, more particularly, to an apparatus for reducing satellite payload complexity when using ground based beam forming.
(b) Description of Related Art
Some satellite communication systems allow multiple ground users to relay data, voice, and other signals over broad geographic regions. In particular, cellular-based satellite communication systems have become especially popular for commercial applications such as telephony because the number of users that can be accommodated within a given geographic region can be greatly expanded beyond that provided by conventional single channel communication systems.
Cellular communications utilize multiple beam antennas and multiple satellite channels to sub-divide a geographic region into a plurality of individual cells. Each cell within the geographic region may be assigned a frequency sub-band selected from a predetermined set of frequency sub-bands. Assignment of the frequency sub-bands typically follows a repeating pattern that results in no two adjacent cells having the same frequency sub-band assignment. This reuse of frequency sub-bands greatly expands the number of users that can be accommodated within a given geographic region. In a typical telephony application a bandwidth B may be divided into p sub-bands each having a bandwidth of B/p. If a region is sub-divided into Nxe2x80x2 cells then each sub-band can be reused Nxe2x80x2/p times. Thus, the total number of users that can be accommodated within the region is (Nxe2x80x2/p)*(B/b), where b is the bandwidth of an individual user and the quantity Nxe2x80x2/p represents the expansion factor provided by using a cellular-based communication system. For example, If Nxe2x80x2=154, p=4, B=30 MHz, and b=6 KHz then the expansion factor equals 38.5 and the total number of possible users equals 192,000. In contrast, a single channel communication system covering the same region would have an expansion factor of 1 and could only accommodate 192,000/38.5 or approximately 5000 users.
To meet system interference requirements, cellular-based communication systems depend on low crosstalk between nearby cells that have been assigned the same frequency sub-band. In practice, the multiple beam antennas that are often used in cellular systems utilize a single reflector or lens and a group of tightly packed feeds in the antenna""s focal plane. These tightly packed feeds create an inherent interference problem because the individual feeds have sidelobes exceeding system interference requirements. Several approaches to reducing this interference to acceptable levels are generally known in the art.
The effects of inter-cell interference due to imperfect feed characteristics can be reduced to acceptable levels by using beam forming systems. Beam forming systems synthesize an improved cell antenna pattern, having the required interference properties, by combining the weighted outputs of multiple feeds. These beam forming systems are usually on board the satellite or within a ground station of the cellular system.
Space-based beam forming systems perform the beam forming on board the satellite. Thus, the hardware associated with spaced-based beam forming systems is carried as a payload on board the satellite. There are several problems with space-based beam forming systems. Namely, conventional beam forming hardware consumes a large amount of electrical power, which is highly undesirable on board a satellite. Furthermore, beam forming hardware is heavy and occupies a significant amount of space, which is also undesirable because it significantly increases satellite production and launch costs.
Ground-based beam forming systems place the beam forming hardware within a ground station of the cellular system. These ground-based systems have the inherent advantage of not being subject to the weight and power restrictions found on board the satellite.
Conventional ground-based beam forming systems transmit individual feed signals from the satellite""s receive antenna to the ground station via an additional gateway frequency band that contains tightly packed sub-bands. In a similar manner, the feed signals for the satellite""s transmit antenna are sent up through another gateway frequency band after the feed signals are generated in the ground station from the beam inputs.
Conventional ground-based beam forming systems have reduced the cost, weight, and power consumption issues associated with having complex, dynamic beam forming hardware on board the satellite by moving this hardware to the ground station. Despite the benefits of ground-based beam forming, conventional systems still require a large amount of transmit and receive hardware to pass the individual feed signals through gateway uplinks and downlinks to and from the ground station. This transmit and receive hardware is carried on board the satellite as payload and is undesirable because of the weight, cost, and power consumption that it entails. The problem of satellite transmit and receive hardware payload complexity is compounded by the fact that using the aforementioned beam forming techniques to compensate for imperfect feed characteristics requires more feeds than would be required if feed patterns were ideal. For example, a circular region divided into Nxe2x80x2 cells would require N=({square root over (N1)}+{square root over (M)})2 cells, where M equals the number of feeds used in synthesizing each beam. Thus, if Nxe2x80x2=154 and M=50 then N=380 feeds.
Transmit and receive hardware payload complexity, and thus, its weight and cost, is a highly sensitive function of the number of feed signals that are transmitted across the satellite""s gateway links. To properly implement a ground-based beam forming system, each feed signal must be channelized into all of the sub-bands used in the particular cellular system. Furthermore, additional redundant transmit and receive processing channels are typically provided for each feed signal. For example, if the cellular system utilizes four sub-bands and includes two redundant processing channels then each feed signal requires six channels of receive hardware for the user uplinks and six channels of transmit hardware for the user downlinks. Therefore, it can be immediately appreciated that reducing the number of feeds transmitted via the gateway links to and from the beam forming ground station can dramatically reduce the complexity of the satellite""s transmit and receive hardware payload.
In accordance with the present invention, a satellite communications payload includes a set of user feeds coupled to a beam forming network. The beam forming network associates the feeds with a set of beamlets such that there are fewer beamlets than feeds. A multi-channel payload coupled to the beam forming network processes the beamlets.
The invention itself, together with further objects and attendant advantages, will best be understood by reference to the following detailed description, taken in conjunction with the accompanying drawings.